The protective immune response to L. monocytogenes is used to investigate general features of cell-mediated immunity to intracellular bacteria. In BALB/c mice, Listeria-specific CD8+ CTL have been isolated which are Kd restricted. Peptides from listeriolysin O (LLO) a molecule secreted in the phagocytic vacuole, and p60, a molecule that is abundantly secreted in the cytoplasm, have been identified as CTL targets. CTL frequency analysis shows that the LLO-derived determinant, LLO 91-99, is immunodominant. We hypothesize that secretion of LLO within the phagocytic vacuole is a contributing factor to the immunodominance of the LLO 91-99 peptide. In Specific Aim I, studies are described that will determine if secretion of defined peptides within the phagocytic vacuole is a contributing factor to their immunodominance. For these studies, we will use L. monocytogenes mutants in which the anchor residue at position 2 within the Kd-binding nanomer peptide (normally a tyrosine) has been replaced by phenylalanine. The mutants retain the wildtype phenotype, yet following immunization with these strains, specific CTL are not stimulated to the mutant peptide sequence. We will use these mutants to deliver specific Listeria-derived peptides to the phagocytic vacuole or the cytoplasm of the L. monocytogenes infected cell in order to determine the influence of these initial locations on the subsequent CTL response. L. monocytogenes has been proposed as a vector to deliver candidate antigens and stimulate specific cell-mediated immunity. The value of vaccine vectors for delivery of candidate antigens depends in part on the ability to specifically stimulate the immune response in an environment of existing anti-vector immunity. In Specific Aim II, we will determine the utility of L. monocytogenes as a vaccine vector and assess the development of peptide specific immunity in animals that have defined levels of existing immunity to the vaccine vector. We will assess the development of peptide specific effector cell and memory cell responses. The results from these studies will guide strategies for the general development of novel vaccine carriers as well as to define the limits of their use within a population whose immunologic status to the vaccine carrier is unknown.